1. Field of the Invention
The present invention is directed to an optical TDM/TDMA (time division multiplex/time division multiple suitable for use in a communications system, and to a method for constructing and operating such a TDM/TDMA system.
2. Description Of the Prior Art
Optical TDM/TDMA systems are known from "Optische Ubertragungstechnik f ur fl achendeckende Teilnhmeranschl usse", No. 04/1992, Section 5, pp. 9-12, No. 09/1992, Sections 9.0 and 9.1, pp. 24-29 and No. 10/1992, Sections 11.0 on pp. 4-6 and "Opal 94", pp. 11-13 of the periodical "Der Fernmelde-lngenieur" (Verlag f ur Wissenschaft und Leben, Georg Heidecker GmbH, Erlangen).
What is referred to as the TDM/TDMA principle is often used in optical transmission systems for wide-area subscriber lines. A central station thereby sends message signals and control signals in time-division multiplex (TDM) mode in a defined cycle via a passive optical network that, as may be seen from FIG. 1 herein, splits in the form of a light waveguide tree structure in the direction from the central station OLT, to subscriber-proximate, detached (remote) units ONU. Each remote unit is connected to a combination of "plain old telephone system" (POTS) units and/or an integrated services digital network (ISDN). In the opposite direction, these detached units ONU transmit signal bursts containing time-division multiplex signals with the same cycle back to the central station OLT in time-division multiplex multiple access (TDMA), i.e. periodically in a preselected sequence in specific time slots. Separate light waveguide tree structures can thereby be employed for the transmission of the signals from the central station OLT and for the opposite direction; in view of reducing outlay, however, operation in full duplex mode is often accomplished with wavelength-division multiplex and only one light waveguide tree structure referred to as a "passive optical network (PON)". Compared to the reception time of the TDM signals, the detached ONU units respectively return their signal bursts to the central station OLT chronologically offset with a delay value that is individually calculated for each detached ONU unit. The delay value is selected for each detached unit ONU such that the signal bursts of all detached units ONU arrive overlap-free and frame-synchronized at the central station OLT after passing through the various fiber lengths to the central station OLT. The control and triggering of the delay thereby ensues by the central station OLT on the basis of control signals co-communicated within the time-division multiplex signals.
A frame format that is constructed basically the same for both transmission directions and is composed of a plurality of successive signal blocks is selected in a known way for the signal transmission. A single signal block is shown in FIG. 2, line 1. The signal block contains a control data part, also referred to as frame overhead (FROH), provided with the address of the detached unit ONU being addressed, this being followed by a comparatively substantially longer useful data part ND. Control commands can be sent from the central station OLT to the individual, detached units ONU in the control data part (FROH). During a calibration (set-up or "commissioning") procedure and for checking the running time during operation, signals referred to as test packet signals that contain a synchronization sequence for measuring transit time are transmitted from the detached units ONU. The acknowledgement signals for the control commands of the central station OLT as well as status information of the detached unit ONU can be transmitted from the detached unit ONU to the central station OLT.
The useful data for the respective detached unit ONU are transmitted from the central station OLT in the useful data part and the useful data of this detached unit ONU for the central station OLT are transmitted in the opposite direction. The transmission of control data thus reduces the transmission of useful data, so that attempts are made to keep the control data part FROH of the signal blocks as small as possible. This control data part, however, also serves for the transmission of the test packet signals in the periodically ensuing calibration or checking of such a TDM/TDMA system, so that the control data part--as set forth below--cannot be arbitrarily shortened.
For explanation, the calibration procedure of two detached units ONU1, ONU5 of FIG. 1 shall be considered, these being arranged at different distances from the central station OLT. It is assumed that the detached unit ONU1 is immediately adjacent to the central station OLT, so that a signal block transmitted from the central station OLT according to line Z1 of FIG. 2 arrives in the detached unit ONU1 after a negligible transit time as indicated by line Z2. The reaction time to commands of the central station OLT is set equal to a first basic delay GD1 by additional delay elements for all detached units ONU, so that the detached unit ONU1 generates a signal block according to line Z3 in response to a command of the central station OLT after this basic delay, this signal block according to line Z3 containing a test packet of measurement signals that are arranged at the end of the control data part, for measuring the transit time for the calibration procedure. This signal block is returned to the central station OLT and arrives thereat after a negligible transit time according to line Z4.
An anticipation window EWF is provided in the central station OLT, having a length corresponding to the length of the control data part FROH of the signal blocks and beginning with the end of the transmission of the control data part by the central. The test packets must arrive in this anticipation window during the calibration procedure and during periodic checking. The beginning of the anticipation window EWF for the test packets thus arises from the minimum signal running time between the central station OLT and the closest detached unit ONU, plus the first basic delay GD1 that has been set. A transit time correction LZK must thus be provided in the first detached unit ONU1 immediately adjacent to the central OLT for supplementing the signal running time on the optical fiber so that the signal block of this detached unit arrives in the central station OLT in the specified position corresponding to line 25.
Line Z6 shows a signal block that arrives after maximum signal running time corresponding to the detached unit ONU5 located at the maximum distance from the central station OLT. After the first basic delay GD1, a signal block corresponding to line Z7 is returned from the detached unit ONU5 to the central station OLT, this signal block arriving exactly in the anticipation window EWF of the central station OLT together with the control data part corresponding to line Z8. A comparison to the specified position for the signal block according to line Z9 shows that, by contrast to line Z5, a correction of running time is not required and that, moreover, the test packets of the control data part would no longer be received in the anticipation window given an even longer running time. An increase in the range of this system thus would require a larger anticipation window, or a longer control data part of the signal blocks, which would be at the expense of the useful data transmission. As a result, however, the transmission capacity for the useful data would be diminished in the overall system and the data running time in the system and the outlay would be increased because of the larger buffer memory for the useful data that would then be required.
An increase in the range is easily possible if no detached units ONU immediately adjacent to the central are present, i.e. the range of distances is not increased. In this case, the differences in the distance between central station OLT and detached units ONU would only lead to differences in the signal running time that, however, do not require an expansion of the anticipation window of the central station OLT.